A New Path Selection Algorithm for MPLS Networks Based on Available Bandwidth Estimation

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Based on Available Bandwidth Estimation



Tricha Anjali1, Caterina Scoglio1, Jaudelice C. de Oliveira1, Leonardo C. Chen1, Ian F. Akyildiz1, Jeff A. Smith2, George Uhl2, and Agatino Sciuto2

1 Broadband and Wireless Networking Laboratory, School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332 USA

{tricha, caterina, jau, leochen, ian}@ece.gatech.edu

2 NASA Goddard Space Flight Center, Greenbelt, MD 20771 USA

{jsmith, uhl, asciuto}@rattler.gsfc.nasa.gov

Abstract. A network should deploy QoS-aware path selection algorithms for ef-ficient routing. To this end, measurements of the various characteristics of the network can provide insight into the state and performance of the network. In this paper, we present a new QoS-aware path selection algorithm for flows re-quiring bandwidth guarantees based on an estimation algorithm for the available bandwidth on the links of the network. The estimation algorithm for the available bandwidth predicts the available bandwidth and also tells the duration for which the estimate is valid with a high degree of confidence. Thus, the path selection algorithm is adaptive and not very computationally intensive.

Keywords: QoS-aware path selection, Passive measurement, Available bandwidth.

1

Introduction

In recent years, there has been a tremendous growth in the Internet. New applications present new traffic patterns and new Quality of Service (QoS) requirements. A QoS-aware path selection algorithm is needed to improve the satisfaction of the user QoS as well as for improved Traffic Engineering (TE). An efficient path selection procedure enables a network operator to identify, for a new flow, a path with sufficient resources to meet the flow’s QoS requirements. These requirements are typically specified in terms of bandwidth guarantees. Thus, the path identification involves some knowledge of the availability of resources throughout the network. Thus, understanding the composition and dynamics of the Internet traffic is of great importance. But observability of Inter-net traffic is difficult because of the Inter-network size, large traffic volumes, and distributed administration. The main tools towards ensuring appropriate QoS satisfaction for all This work was supported by NASA Goddard and Swales Aerospace under contract number

S11201 (NAS5-01090)

Corresponding Author. Research Assistant, 250 14th Street, Suite 556, Atlanta, GA 30318, USA Phone: +1-404-894-6616 Fax: +1-404-894-7883

B. Stiller et al. (Eds.): QofIS/ICQT 2002, LNCS 2511, pp. 205–214, 2002. c

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applications will be measurements of the network and appropriate path selection proce-dures.

Without loss of generality, we assume in this paper that a source node is presented with a request to reserve bandwidth resources for a new flow with specific QoS require-ments, and is responsible for finding a suitable path to the destination. This is similar to the source routing model. Also, we assume the link state information model, where a network topology database is available for state information about nodes and links in the network. The state information is obtained by measurements from the network elements. This information can then be used by the path selection process.

A network can be monitored either in an active or passive manner. First gives a measure of the performance of the network whereas the latter of the workload on the network. Both have their merits and should be regarded as complementary. The active approach relies on the capability to inject packets into the network and then measure the services obtained from the network. It introduces extra traffic into the network. But the active approach has the advantage of measuring the desired quantities at the desired time. Passive measurements are carried out by observing normal network traffic, without the extra load. The passive approach measures the real traffic. But the amount of data accumulated can be substantial because the network will be polled often for information.

The rest of the paper is organized as follows. In Sect. 2, we present the importance of available bandwidth and a description of various bandwidth measurement techniques. This is followed by a description of our path selection algorithm in Sect. 3. In Sect. 4, we present our algorithm for available bandwidth estimation. In Sect. 5, the results of the experiments with the available bandwidth estimation algorithm are presented. Finally, we conclude in Sect. 6.

2

Measuring Available Bandwidth

There are various quantities of interest that can be insightful about the state of the net-work. Available bandwidth (together with latency, loss etc.) can predict the performance of the network. Based on the bandwidth available, the network operator can obtain in-formation about the congestion in the network, decide the admission control, perform routing etc. For MPLS networks, the available bandwidth information can be used to decide LSP setup [1], routing (Shortest Widest Path [2], Widest Shortest Path [3]), LSP preemption [4], etc. Several applications could benefit from knowing the available bandwidth of an LSP. One such application is congestion control. Available bandwidth information can also be used to build multicast routing trees more efficiently and dy-namically.

It is desirable to obtain the available bandwidth information by measurements from the actual LSPs because they give more realistic information about the available band-width. The available bandwidth information can also be obtained by subtracting the nominal reservation for the tunnels from the link capacity which gives a lower bound. In this paper, we will use the terms LSP and link interchangeably because an LSP is equivalent to links in an MPLS network. The path between two LSRs can be composed of multiple LSPs.

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The available bandwidth on a link is indicative of the amount of additional load that can be routed over the link. Obtaining an accurate measurement of the available bandwidth can be crucial to effective deployment of QoS services in a network. Avail-able bandwidth can be measured using both active and passive approaches. Various tools to measure available bandwidth of a link in the network are available. The bot-tleneck bandwidth algorithms can be split into two families: those based on pathchar [5] algorithm and those based on Packet Pair [6] algorithm. The pathchar algorithm is an active approach which leads to the associated disadvantages of consumption of significant amount of network bandwidth etc. The packet pair algorithm measures the bottleneck bandwidth of a route. It can have both active and passive implementations. In [7], the authors have proposed another tool to measure bottleneck link bandwidth based on packet pair technique. None of these tools measures the available bandwidth or utilization of a desired link of a network. In [8], the authors have proposed an active approach to measure the available bandwidth of a route which is the minimum available bandwidth along all links of the path. Another active approach to measure the through-put of a path is Iperf [9] from NLANR that sends streams of TCP/UDP flows. Cisco has introduced the NetFlow technology that provides IP flow information for a network. But in a DiffServ environment, the core of a network is interested in aggregate rather than per-flow statistics, due to the scalability issues.

All the tools, except NetFlow, give path measurements based on an active approach. A network operator, on the other hand, would be interested in finding the available band-width on a certain link of the network. He does not need the end-to-end tools that utilize the active approach of measurement. One solution is to use Simple Network Manage-ment Protocol (SNMP) which is a short-term protocol to manage nodes in the network. Its operation revolves around two key components: managed devices and management systems. Managed devices store management information in Management Information Bases (MIBs) and make this information available to the management systems. Thus SNMP can be used as a passive technique to monitor a specific device. MRTG [10] is a tool based on SNMP to monitor the network links. It has a highly portable SNMP implementation and can run on most operating systems.

Thus, to obtain the available bandwidth on a certain link of the network in a pas-sive manner whenever he desires, the network manager can use MRTG. But MRTG has the limitation that it gives only 5 minute averages of link utilization. For appli-cations like path selection, this large interval averaging may not be enough. We have modified MRTG to MRTG++, to obtain averages over 10 second durations. This gives the flexibility to obtain very fine measurements of link utilization. We utilize a linear regression-based algorithm to predict the utilization of a link. The algorithm is adaptive because a varying number of past samples can be used in the regression depending on the traffic profile.

3

Path Selection Algorithm

In addition to just finding a path with sufficient resources, other criteria are also im-portant to consider during the path selection. They include optimization of network

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utilization, carried load, etc. In the following, we describe the related work followed by the description of the proposed path selection scheme.

3.1 Related Work

Various QoS routing algorithms [3, 11, 12] have been extensively studied in literature. All these schemes utilize the nominal available bandwidth information of the links dur-ing the path selection. As most of the traffic flows do not follow the SLA agreement very closely, the nominal utilization of a link is highly overestimated than the actual instantaneous utilization. This leads to non-efficient network resource utilization.

A few measurement based methods are proposed in literature for resource allo-cation, admission control etc. In [13], a resource allocation method is proposed that estimates the entropy of the system resources based on measurements. A connection admission control algorithm for ATM networks based on the entropy of ATM traffic (obtained from measurements) is given in [14]. Another measurement based admission control framework is given in [15]. A measurement-based routing algorithm is given in [16]. The algorithm tries to minimize the end-to-end delay.

3.2 QoS-aware Path Selection

we propose a path selection scheme that is based on obtaining more accurate link utiliza-tion informautiliza-tion by actually measuring the same from the network elements involved. However, it is not efficient to update the link state database with the available band-width information with very fine granularity, due to scalability concerns. Also, due to the varying nature of the available bandwidth, a single sample of measured available bandwidth does not have much significance. This is because the actual traffic profile may be variable and path selection decisions based on a single sample are more than likely to be wrong. Thus, a method has to be devised that performs an estimation of the link utilization in the future. We have proposed an algorithm for available bandwidth estimation that is explained in next sections.

Once, we obtain the available bandwidth estimates, we can then use these values as the input to a shortest widest path routing algorithm to find an efficient path. Even though there is some inaccuracy involved in path selection based on estimates of avail-able bandwidth, the results in [17] demonstrate that the impact of the inaccuracy in bandwidth information on the path selection procedure is minimal. Our path selection procedure is detailed in Fig. 1. As shown, the available bandwidth estimation algorithm is used only for the links where the estimate has not been calculated before. The

param-eterthreshold in the figure is used as a benchmark for path selection. If the bandwidth

request is more than a certain fraction of the bottlenect link bandwidth,the request is re-jected. This is done to limit the congestion in the network. Once the estimate is obtained, the available bandwidth information can be used in the shortest widest path routing al-gorithm as weights of the links. The new flow will be admitted into the network if the flow’s bandwidth requirement is less than a certain threshold, which is calculated based on the available bandwidth on the path. Optionally, the arrival of the flow can trigger the re-dimensioning/setup of an LSP between the ingress and the egress LSRs. An optimal policy for this procedure is given in [1].

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Path Selection Algorithm:

1. At time instantk, a bandwidth request r arrives between nodes i and j.

2. Run the available bandwidth estimation algorithm links with no bandwidth estima-tion available.

3. Compute the best path using the shortest widest path algorithm with weights as calculated in step 2.

4. Obtain the available bandwidthA on the bottleneck link of the path.

5. Ifr > A ∗ threshold, reject this path and return to step 3. Else, path is selected

for the request.

6. If no path available, request rejected and network is congested.

Fig. 1. The path selection algorithm

4

The Available Bandwidth Estimator for MPLS Networks

Our approach for available bandwidth estimation is based on the use of MRTG. A net-work operator can enquire each router in the domain through SNMP and obtain the information about the available bandwidth on each of its interfaces. The most accurate approach will be to collect information from all possible sources at the highest possi-ble frequency allowed by the MIB update interval constraints. However, this approach can be very expensive in terms of signaling and data storage. Furthermore, it can be redundant to have so much information. Thus we present an approach that minimizes the redundancies, the memory requirements for data storage and the signaling effort for data retrieval.

We can set the measurement interval of MRTG and measure the average link uti-lization statistic for that interval. We define for a link between two nodes i and j:

C: Capacity of link in bits per sec,

A(t): Available capacity at time t in bits per sec ,

L(t): Traffic load at time t in bits per sec,

τ : Length of the averaging interval of MRTG,

Lτ[k], k ∈ N : Average load in [(k − 1)τ, kτ].

The available capacity can be obtained asA(t) = C − L(t). So, it would be sufficient to measure the load on a link to obtain available bandwidth. Note that we have not explicitly shown thei − j dependence of the the defined variables. This is because the

analysis holds for any node pair independent of others. We also define

p is the number of past measurements in prediction,

h is the number of future samples reliably predicted,

Ah[k]: the estimate at kτ valid in [(k + 1)τ, (k + h)τ]. Our problem can be formulated as linear prediction:

Lτ[k + a] = p−1



n=0

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Available Bandwidth Estimation Algorithm:

1. At time instantk, available bandwidth measurement is desired.

2. Find the vectorswa, a ∈ [1, h] using Wiener-Hopf equations given p and the

previous measurements.

3. FindˆLτ[k + 1], . . . , ˆLτ[k + h]Tfrom (1) and[Lτ[k − p + 1], . . . , Lτ[k]] . 4. PredictAh[k] for [(k + 1)τ, (k + h)τ] .

5. At time(k + h)τ, get [Lτ[k + 1], . . . , Lτ[k + h]]T. 6. Find the error vector[eτ[k + 1], . . . , eτ[k + h]]T.

7. Setk = k + h.

8. Obtain new values forp and h.

9. Go to step 1.

Fig. 2. The Available Bandwidth Estimation Algorithm

where on the right side are the past samples and the prediction coefficientswa[n] and on the left side, the predicted values. The problem can be solved in an optimal manner using Wiener-Hopf equations. We dynamically change the values ofp and h based on

the traffic dynamics.

The available bandwidth estimation algorithm is given in Fig. 2. We definep0andh0 as the initial values forp and h. In step 2 of the algorithm, we need to solve the Wiener-Hopf equations. They are given in a matrix form asRLwa= ra,a = 1, . . . , h, i.e.,

   rL(0) · · · rL(p − 1) .. . . .. ... rL(p − 1) · · · rL(0)       wa(0) .. . wa(p − 1)    =    rL(a) .. . rL(a + p − 1)   

In order to derive the autocorrelation of the sequence from the available measurements, we estimate it as rL(n) = 1 N N  i=0 Lτ[k − i]Lτ[k − i − n]

whereN affects the accuracy of the estimation, i.e., more samples we consider, more precise the estimation is. The number of samples needed for a givenn and N is (n + N ). Since the assumption about stationarity of the measurement sequence may not be

accurate, we update the values of the autocorrelation every time we change the value

ofp in step 8 of the algorithm. The solution of the Wiener-Hopf equations will provide

wa that can be used for predicting (k + a), a = 1, . . . , h. Since the matrix RLis

symmetric Toeplitz, Levinson recursion can be used to solve forwaefficiently. For a

sizep matrix, it involves (2p2− 2) multiplications and (2p2− 3p + 1) additions.

From thewa’s, we predict

Lτ[k + 1], . . . , Lτ[k + h]

T

using (1). Next step is to obtain an estimate of the available bandwidth for the interval[(k + 1)t, (k + h)t]. The available bandwidthAh[k] is given as Ah[k] = C −max

Lτ[k + 1], . . . , Lτ[k + h]



for a conservative estimate. As the estimate is valid for a duration longer than the indi-vidual sampling times, the path selection scheme is feasible for applications requiring

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Interval Variation Algorithm:

1. Ifσ/µ > T h1, decreaseh till hminand increasep till pmaxmultiplicatively. 2. IfT h1> σ/µ > T h2, decreaseh till hminand increasep till pmaxadditively. 3. Ifσ/µ < T h2, then:

(a) Ifµ > T h3∗ ME2, decreaseh till hminand increasep till pmaxadditively. (b) IfT h3∗ ME2 > µ > T h4∗ ME2, keeph and p constant.

(c) Ifµ < T h4∗ ME2, increaseh and decrease p till pminadditively.

Fig. 3. Algorithm for h and p

estimates of available bandwidth valid over long durations. Thus, even though LSPs are long-living, our path selection procedure is still valid.

After obtaining the actual load[Lτ[k +1], . . . , Lτ[k +h]]Tat time(k +h)t, we find the prediction error vector[eτ[k + 1], . . . , eτ[k + h]]T where each element is given as

eτ[k + a] =



Lτ[k + a] − Lτ[k + a]

2

for a = 1, . . . , h.

Next, we estimate new values forp and h based on a metric derived from the mean

(µ) and standard deviation (σ) of error eτ. The algorithm is given in Fig. 3.

In the algorithm,MEis the maximum error value and we have introducedhminand

pmaxbecause small values ofh imply frequent re-computation of the regression coef-ficients and large values ofp increase the computational cost of the regression. Also,

we have introduced the thresholdsT h1toT h4 to decide when to change the values of the parametersp and h. They are determined based on the traffic characteristics and

the conservatism requirements of the network domain. The values of these parameters are held constant throughout the network. In other words, they are not decided on a link-by-link basis.

In this section, we have presented an algorithm for estimating the available band-width of a link by dynamically changing the number of past samples for prediction and the number of future samples predicted with high confidence. The objective of the al-gorithm is to minimize the computational effort while providing a reliable estimate of available bandwidth of a link. It provides a balance of the processing load and accuracy. The algorithm is based on the dynamics of the traffic, i.e., it adapts itself.

5

Experimental Results

In this section, we describe the experiments we used to validate and quantify the utility of the available bandwidth estimation algorithm, and thus effectively demonstrating the performance of the proposed path selection algorithm. First, we describe a few implementation details about the estimator.

A manager can retrieve specific information from the device using the Object Iden-tifiers (OIDs) stored in the devices. The in-bound and out-bound traffic counters on the router interfaces can be periodically fetched to calculate the traffic rate. We have mod-ified the MRTG software to reduce its minimum sampling down to 10 seconds. This

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200 250 300 350 400 450 500 550 600 10 15 20 25 30 35 40 Sample number Bandwidth (MB/s) Actual Estimated

(a) Input traffic on Atlanta router

200 250 300 350 400 450 500 550 600 40 50 60 70 80 90 100 110 Sample number Bandwidth (MB/s) Actual Estimated

(b) Input traffic on Cleveland router

Fig. 4. Estimation performance

provides flexibility to the network manager to obtain more precise and instantaneous measurement results. The network manager should decide the optimal measurement period based on the traffic characteristics, the required granularity for the measured values and the appropriate time-scale of the application utilizing the measured values.

Next, we describe the methodology in running the experiments. The algorithm for available bandwidth estimation on a link does not make any assumption about the traf-fic models. The available bandwidth estimation algorithm works based on the mea-surements obtained from the network link. Thus, we do not need a network simulator. Instead, we can apply the algorithm to traffic traces obtained from real networks. In the following, we present the traffic profile predicted by available bandwidth estimation algorithm together with the actual profile. We do not present the predicted available bandwidth profile because that can be calculated by taking the difference of the link capacity and the utilization and thus it does not present significant information, when compared to the predicted utilization.

The choice of the thresholdsT h1,T h2, etc. andhmin,pmax used for updating the values ofp and h in Section 4 has to be made by the network operator depending on the conservativeness requirements of the network operation. We have obtained the following results by choosingT h1 = 0.9, T h2 = 0.7, T h3 = 0.5, T h4 = 0.3 and

hmin = 10, pmax = 50. All the traffic traces used in the following results have been obtained from Abilene, the advanced backbone network of the Internet2 community, on March 13, 2002. In Fig. 4(a), the available bandwidth estimation algorithm is applied to the input traffic on the Atlanta router of the Atlanta-Washington D.C. link. In Fig. 4(b), same is done for the input traffic on the Cleveland router from the Cleveland-NYC link. As we can see, in both cases, the utilization estimation obtained by taking the peak pre-diction provides a conservative estimate. Also, whenhminis increased, the estimation becomes worse (see Fig. 5), in the sense that it does not follow the sequence closely but is still very conservative. We propose the use of overestimation as a metric to quantita-tively measure the performance of the scheme for available bandwidth estimation. For

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200 250 300 350 400 450 500 550 600 10 15 20 25 30 35 40 Sample number Bandwidth (MB/s) Actual Estimated

Fig. 5. Input traffic on Atlanta router (hmin= 20)

the case in Fig. 4(a), the mean overestimation is 1.31 MB/s for the estimation. Similar values are obtained for the case depicted in Fig. 4(b).

When compared with MRTG, we provide the available bandwidth estimates less frequently without a large compromise in the reliability of the estimate. In other words, the utilization profile obtained as a result of MRTG coincides with the actual traffic pro-file in Figs. 4(a) and (b), but our algorithm provides an estimate of the link utilization which is nearly accurate. Overall, our path selection algorithm has a higher computa-tional complexity than a simple scheme based on nominal link utilizations. However, in that case, the links will be underutilized as the user agreements are based on the maximum traffic profile.

The performance of the proposed path selection algorithm is better than the shortest widest path routing algorithm because the available bandwidth estimation algorithm provides more accurate estimates of the link available bandwidth than just taking an instantaneous sample. In the worst case, when the link traffic is highly variable, the prediction interval for the available bandwidth estimation algorithm is small. In effect, it is similar to taking instantaneous samples. Thus, the worst case performance of the path selection algorithm is similar to the shortest widest path routing algorithm.

6

Conclusions

We have presented an algorithm for path selection in MPLS networks. The algorithm is based on estimation of the available bandwidths on links of MPLS networks. The available bandwidth estimation algorithm predicts the available bandwidth and tells the duration for which the estimate is valid with a high degree of confidence. The available bandwidth estimation algorithm dynamically changes the number of past samples that are used for prediction and also the the duration for which the prediction holds. The path selection algorithm utilizes these estimates as weights in the shortest widest path routing algorithm. In general, the performance of the path selection algorithm is better than the shortest widest path routing algorithm. In worst case, the path selection algorithm gives a performance similar to shortest widest path routing algorithm.

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